The level indicating and elevation indicating system of the present invention is an improvement and an extension of the device and apparatus of my prior U.S. Pat. Nos. (4,231,163 and 3,849,898).
Mechanical methods for detecting when the surface of a fluid, or a float supported by fluid, has reached a specified level are known. Such methods require part of the buoyancy force of a float to be used to actuate a switch. This may be done by a float moving a magnet close enough to a magnetically actuating switch or by pushing against switch contacts. Such mechanical methods have the characteristic low repeatability and hysteresis of force actuated switches made worse by the inherent elasticity of a float supported by fluid. Additionally, they must be protected from the dirt, dust, and corrosive vapors likely to be present.
Electric methods for detecting when the surface of a fluid has reached a specified level are known. Such methods act by using the decrease of resistance between electrodes that penetrate into the hose when the electrodes are covered by a conducting fluid. For such methods to operate, the fluid used must be conductive, the electrodes must be able to be insulated from each other, and the resistance between the electrodes and the fluid must stay within limits. Since many low cost conductive fluids tend to be chemically active (such as water plus acid or water plus a metallic salt) one must use some care in handling the fluids and one will prefer to use electrodes of a noble metal. U.S. Pat. No. 4,434,561 teaches the use of a fluid having at least the conductivity of "tap water" when the electrodes are used with an amplifier. The same patent shows the characteristic of such an electric method to have a single indicator.
At best, the known methods may be able to indicate either that the level of the surface of a fluid is within some small distance of, or below, a reference or, in the alternative, that the level of the surface of a fluid is within some small distance, or above, a reference. There is therefore a need in the art to provide a method for accurately determining when the level of the surface of an arbitrary fluid is within a certain small distance of a reference, above the same certain small distance of a reference, or below the same certain small distance of a reference. Without such a last described method, the overall accuracy and repeatability of such systems is significantly limited.
It has been known to sight across the surface of a fluid to a target point. Such surfaces are not flat, but have significant curvature due to the surface tension of the fluid and the lack of "wetting" of many fluids. This results in some uncertainty as to what the level of the surface is and some parallax error. There is therefore a need in the art for a method of accurately determining a horizontal plane that is a fixed distance from the surface of the fluid.